code: def mean_squared_error(y, y_pred):
      return np.mean((y - y_pred) ** 2)
  
file_path: GPflow/notebooks/tailor/external-mean-function.pct.py
imports: import matplotlib.pyplot
  import numpy
  import tensorflow
  import gpflow
  from gpflow.ci_utils import reduce_in_tests
  from gpflow.kernels import RBF
  from gpflow.models import GPR
  from gpflow.config import default_float
  from gpflow import set_trainable
  import time
label_desc: Use image node_id 8 for calling a global function with example usage: mean_squared_error(y, y_pred) and returns: np